The present invention relates to a gun drill which is used in drilling a long bore in, for example, steel and, more particularly, to a gun drill improved to enable drilling with a greater feed rate.
A. Description of the Related Art
A gun drill of the kind described, in particular a gun drill suitable for a large feed rate, is enclosed in Japanese Utility Model Laid-Open No. 4-70418 of the same applicant as this application.
In this gun drill, a cutter tip brazed to a shank has a cutting edge formed on the end thereof, wherein the cutting edge has a curved cutting edge portion which is adjacent to the axis of rotation and which is convex in the forward direction of rotation when viewed in the axial direction from the side adjacent to the free end of the cutter tip, a radially outermost straight cutting edge portion which is on an imaginary plane containing the axis of rotation, and a concave edge portion which is intermediate between the straight cutting edge portion and the curved cutting edge portion, the concave cutting edge portion including a slant portion connected to the straight cutting edge portion and slanted backward as viewed in the rotation direction towards the axis of rotation and a concave portion which interconnects the radially inner end of the slant portion and the convex curved cutting edge portion. The height of the convex curved cutting edge portion from the above-mentioned imaginary plane is determined in the relation to the diameter of rotation of the outer peripheral end of the straight cutting edge portion, i.e., the diameter D of the gun drill, such as to fall within the range between 0.03.times.D and 0.06.times.D. At the same time, the radial width of the portion of the convex curved cutting edge portion projecting forward toward the rotation direction beyond the imaginary plane is determined to range between 0.25.times.D and 0.35.times.D. Furthermore, the angle formed by the tapered portion of the concave cutting edge portion is determined to range between 5.degree. and 30.degree.. According to this arrangement, it is possible to promote the discontinuation or severance of the cut metal chip while suppressing an increase in the cutting resistance, thus achieving a greater feed of the drill.
Table 1 shows the results of a test drilling conducted by using a gun drill which is disclosed in the above-mentioned Utility Model Laid-Open Publication. In this table, a mark O indicates that the drilling was preformed in a good manner without any trouble, a mark .DELTA. indicates that the drilling was rather unstable due to vibration or seizure of the cut metal chips although the drilling was not impossible, and a mark x indicates that the drilling was materially impossible due to clogging with the cut metal chips or increase in the cutting resistance. The test drilling was carried out at a cutting rate of 70 m/min, using SCM 440 (HB 260) as the material of the work subjected to the drilling.
TABLE 1 ______________________________________ 12 .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .DELTA. X 10 .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .DELTA. .DELTA. X 8 .largecircle. .largecircle. .largecircle. .largecircle. .DELTA. .DELTA. X 6 .largecircle. .largecircle. .largecircle. .DELTA. .DELTA. X Cutting edge 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 dia. Feed (mm/rev) ______________________________________
From Table 1, it is understood that the gun drill of the type disclosed in the above-mentioned Publication can perform good drilling even at a large feed rate of 0.12 mm/rev when the cutting edge diameter (diameter D of the gun drill) is 12 mm, and a feed rate of 0.06 mm/rev can be maintained even when the gun drill diameter D is as small as 6 mm. In contrast, the conventional gun drill, which was known prior to the filing of the application of the above-mentioned Utility Model, showed a lack of stability in the drilling when the feed rate exceeded 0.06 m/rev, when the diameter of the cutting edge was 12 mm or so. Thus, the gun drill shown in the above-mentioned Publication exhibits an improved cutting performance over the conventional gun drill.
B. Problems Solved by the Invention
In recent years, there has been an increasing demand for gun drills which can perform boring at further increased feed rates, in order to cope with current requirements for higher efficiency and higher speed of machining.
The present inventors have conducted various experiments with a view to achieving a higher feed rate based on the known drill disclosed in the above-mentioned Japanese Utility Model Publication of the same applicants. As a result, the present inventors have discovered that, by enabling more efficient severance of a cut metal chip and control of the direction of the flow of the cut metal chip, a cut metal chip which is generated at a greater rate during machining at a higher feed rate can be smoothly discharged, thus preventing an increase in the cutting resistance attributable to clogging of the cut metal chip, as well as preventing seizure of the cut metal chip attributable to a rise of the temperature in the bore which is being drilled, thereby enabling drilling at a greater feed rate.
The present invention is based upon the above-described discovery and is aimed at providing a gun drill which can operate with a greater feed rate than the above-described known gun drill, thereby achieving further improvement in the cutting performance.